Introduction

Acute skeletal muscle wasting is a major contributor to post critical illness physical impairment. However, the bone response remains uncharacterized. We prospectively investigated the early changes in bone mineral density (BMD) and fracture risk in critical illness.

Methods

Patients were prospectively recruited ≤24 hours following intensive care unit (ICU) admission to a university teaching or a community hospital (August 2009 to April 2011). All were aged >18 years and expected to be intubated for >48 hours, spend >7 days in critical care and survive ICU admission. Forty-six patients were studied (55.3% male), with a mean age of 54.4 years (95% confidence interval (CI): 49.1 to 59.6) and an APACHE II score of 23.9 (95% CI: 22.4 to 25.5). Calcaneal dual X-ray absorptiometry (DXA) assessment of BMD was performed on day 1 and 10. Increase in fracture risk was calculated from the change in T-score.

Rapid and early muscle wasting contributes to the significant long-term functional impairment observed in survivors of critical illness [1-4]. Although muscle and bone mass correlate in healthy individuals [5-7], there are limited data reporting the impact of critical illness on bone health. During acute critical illness, mechanical unloading as a consequence of bed rest [8], inflammation [9], acidaemia [10], vitamin D deficiency [11], corticosteroid use [12] and hypoxia [13] may all worsen bone health and reduce bone mineral density (BMD). Indeed, markers of bone turnover increase during critical illness [11,14]. Such bone demineralization may explain symptoms of generalised musculoskeletal pain in survivors of critical illness [1,2], and the reported increase in fracture risk in elderly females following critical illness [15]. However, the bone response to critical illness has never before been prospectively studied. We thus performed a pilot study to investigate the early effects of critical illness on BMD and fracture risk, and also sought clinical factors that might be associated with early bone demineralization.

Subjects comprised a subgroup of the Musculoskeletal Ultrasound in Critical Illness: Longitudinal Evaluation cohort (trial registered with Clinicaltrials.gov, identifier: NCT01106300) [4]. Ethical approval was obtained from the University College London ethics committee. Patients were recruited within 24 hours of admission to a university hospital (Kings College Hospital NHS Foundation Trust) or a community hospital intensive (Whittington Hospital NHS Trust) care unit (ICU) between August 2009 and April 2011. All were anticipated to be invasively ventilated for more than 48 hours, spend more than 7 days in the ICU and survive their ICU stay. Patients who were pregnant or suffering lower limb amputation, primary neuromuscular disease or cancer were excluded. At enrolment, written assent was obtained from the next-of-kin, with retrospective patient consent obtained when full mental capacity was regained.

BMD was assessed using dual X-ray absorptiometry (DXA) imaging on day 1 and day 10 (DXL Calscan, Demetech AB, Sweden), which has a coefficient of variance of 0.9%. Fracture risk was calculated from a change in T-score whereby the relative risk of a major osteoporotic fracture increases 1.5-fold (95% confidence interval (CI): 1.4 to 1.6) per standard deviation below the mean T-score [16]. Detailed clinical and physiological bedside data were collected, as previously described [4].

Statistical analysis

All data were assessed for normality using D’Agostino and Pearson omnibus normality tests, and analyzed using Student’s t-test, Pearson’s coefficient, Mann-Whitney U test and Wilcoxon’s signed rank tests, as appropriate. For the purposes of hypothesis generation in this pilot study, we sought a parsimonious model of associated physiological factors. Age, sex and chronic disease were forced into bivariable logistical regression (Statistical Package for the Social Sciences version 17; SPSS Inc., Chicago, IL, USA) using a threshold of 2% loss of BMD, which is twice the expected loss from bed rest alone [8]. Statistically significant independent variables from the bivariable analysis were entered into a backward multivariable analysis if the P value was 0.10 or less.

Fifty-seven patients assented to serial DXA scanning. Of these, seven did not survive ten days, one was transferred to another hospital, one withdrew from the study, one was discharged before day ten and one was unable to have serial scans for technical reasons. Forty-six patients were included in the final analysis. The characteristics of these 46 patients, shown in Table 1, did not differ from those withdrawn, except for a higher Simplified Acute Physiology Score (SAPS II) (40.9 (95% CI: 37.5 to 44.3, n = 46) versus 53.3 (95% CI: 44.2 to 62.4, n = 10); P <0.01). Four patients had pre-morbid conditions associated with possible disrupted calcium homeostasis (one with hypothyroidism, one with Crohn’s disease, two with hyperthyroidism). Their baseline DXA measurement was no different from the remaining cohort (0.387 ± 0.07 versus 0.437 ± 0.01, P = 0.350). Twenty eight patients (61%) were defined as having osteopenia on day one of the study; however, no patients received renal replacement therapy using citrate anticoagulation with calcium replacement, and no subjects received regular selective serotonin reuptake inhibitors or serotonin norepinephrine reuptake inhibitors (associated with lower BMD) prior to critical illness. Further data regarding recruitment and survival, as well as baseline laboratory values, are available in Additional file 1.

Table 1

Characteristics of patients who had serial measurements versus those who only had admission measurements

Clinical relevance

The relationship between loss of BMD and rise in serum calcium concentration is to be expected. However, rapid early bone demineralization, of similar magnitude to that observed after much more prolonged weightlessness in space [18], occurs rapidly and early in critically ill patients with ARDS. The association of ARDS with bone demineralization is physiologically plausible: ARDS results in systemic release of inflammatory cytokines [19] such as interleukin-6 [20], TNFα [20,21], interleukin-1 [21] and interleukin-8 [22], which stimulate osteoclastogenesis and bone resorption with calcium mobilized into the circulation from these bone stores [23]. The scale of this loss is associated with an increase in calculated prospective fracture risk; an issue of some importance given that ARDS survivors have a number of independent risk factors for falls [24], including accelerated skeletal muscle wasting [4] and marked loss of executive function [25]. Bone is certainly able to respond rapidly to remodelling forces: changes are observed within 15 to 21 days in rodent models of unloading [26,27], and markers of bone turnover alter with single bouts of exercise in humans [28]. However, we are unaware of studies of the human skeletal response to illness over timeframes a short as that which we have addressed. Whilst in vitro evidence exists that hypoxia is detrimental to skeletal physiology and has both an inhibitory effect on osteoblastogenesis [29] and an activator-enhancing effect on osteoclastogenesis [30], no relationship was seen with admission hypoxia, although intermittent hypoxia as a stimulus cannot be excluded.

Limitations

Whilst acknowledging that the first day of ICU admission is not the first day of critical illness, the median time to ICU admission was 24 hours, and 23 patients were admitted after a sudden acute event (for example trauma, myocardial infarction or intracranial bleed) with no antecedent decline. Nonetheless, these data should be considered hypothesis-generating. The limited sample size also precludes detailed exploration of other risk factors, such as osteopaenia, and generalisation to specific patient groups. Regrettably, we are unable to determine whether the observed changes translated into clinical skeletal events, given that this pilot study was not funded for the post-discharge follow-up of patients. Larger observational cohort studies, with extended follow-up periods, are required to determine the impact of critical illness on BMD and actual fracture risk. The determination of calcaneal BMD using DXA is valid and comparable to hip and spine DXA in determining fracture risk [16,31], and is more readily performed than the assessment of spine and hip bone densitometry in a remote imaging facility during the early stage of critical illness when the patient is most unstable. The coefficient of variance of DXA measures was 0.9%. A loss of more than 2% in calcaneal BMD (as we sought) was thus likely to represent true loss as opposed to measurement error.

Rapid bone demineralization, associated with an increase in fracture risk, was observed in critically ill patients with ARDS. More extensive and extended hypothesis-driven epidemiological cohort studies are required to confirm this finding, and to determine whether bone demineralisation represents a new therapeutic target in reducing morbidity following critical illness.

Acknowledgements

ZP is funded by the National Institute of Health Research (NIHR) UK. This research was supported by the NIHR University College London Hospitals Biomedical Research Centre (BRC). Additional funding was received from the European Society of Intensive Care Medicine, Guy’s and St Thomas’ and King’s College London NIHR Comprehensive Biomedical Research Centre and the Whittington Hospital NHS Trust. MM is funded by the Wellcome Trust UK and acknowledges the BRC at Imperial College and King’s College London for infrastructure support. This paper was supported by Grant Number, DRF-2010-03-114.

The NIHR, the European Society of Intensive Care Medicine, Guy’s and St Thomas’ and King’s College London NIHR Comprehensive BRC and the Whittington Hospital NHS Trust had no role in the design and conduct of the study; collection, management, analysis and interpretation of the data; preparation, review or approval of the manuscript; or decision to submit the manuscript for publication.

The authors are grateful to the patients and staff at both King’s College Hospital and the Whittington Hospital NHS trust, without which this study could not have been performed. ZP has full access to all data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

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